Engineering components generally fail in one or a combination of three basic modes: corrosion, wear and fracture. The requirements for material properties to combat each of these modes are different and often conflicting. In many cases, a monolithic bulk material can only provide a good compromise to satisfy the differing and conflicting requirements. One effective means of mitigating against damage, especially damage due to corrosion and wear, is to modify the composition and/or microstructure of the surface and/or near-surface region of the component to improve both mechanical properties and resistance to failure.
Among the many technologies available to treat the surface and/or near-surface of a component, surface modification during a casting process has many distinct advantages. Surface modification during casting generally permits formation of thick strengthening layers, choice of a wide assortment of materials, strengthening of specifically selected areas, application to large components or complicated shapes, reduction of overall process cost, and easy process implementation. However, using currently available casting surface modification techniques, the surface strengthening layers are prone to defects and it is difficult to achieve accurate dimensions and smooth surface finishes. Additionally, in some applications, thick alloying layers cannot be easily applied.
In the art, surface modification during casting is normally done by placing certain types of special material, which are normally powders or particles, in a casting mould at certain areas before casting. Examples of the special material include powders or particles of metals/alloys, oxides, nitrides, carbides, mixtures of ceramics with metals/alloys, mixtures of cermets with metals/alloys, mixtures thereof, etc. When a liquid casting material, e.g. molten metal, is cast into the mould and solidifies, a special strengthening layer is formed on the surface of the casting at a region corresponding to the area of the mould where the special material was placed. Methods of placing the special material in the casting mould can be generally divided into two categories: (1) methods that involve the use of a binder; and, (2) methods that do not use a binder.
Methods (1) of placing the special material in the casting mould that involve the use of a binder include, for example: (a) forming a paste by mixing the special material with an organic or inorganic binder and coating the paste on a surface of the casting mould where required; (b) mixing the special material with a binder, shaping the mixture into a pre-form, placing the pre-form in a certain area of the casting mould, and then casting with or without pressure; (c) pre-treating a pattern by coating it with a paste or enclosing powders in certain areas of the pattern before making the casting mould (lost pattern method); (d) applying a high temperature adhesive to the surface of the casting mould and then applying the special material to the adhesive. One of the most apparent problems with methods involving the use of a binder is that high heat used during the casting process causes binder decomposition leading to defects in the casting, for example inclusions and gas porosities.
Methods (2) of placing special material in the casting mould that do not involve the use of a binder include, for example: (a) enclosing the special material in a holding container having perforated openings and placing the container at required positions in the casting mould cavity; (b) placing ferromagnetic powders at a certain area of the casting mould and using magnetic forces or magnetic forces combined with vacuum to hold the ferromagnetic powders in position before and during casting; (c) applying loose powders/particles of the special material onto the surface of the casting mould and casting with applied pressure or under ambient pressure; (d) applying a layer of the special material on to required areas of the surface of the casting mould by spraying (e.g. thermal spraying); (e) applying a vacuum, with the help of a thin plastic film, to hold loose particles or powders of the special material on the surface of the casting mould before casting.
Methods that do not involve the use of a binder potentially allow for the production of better quality surface strengthening layers. However, there are some limitations on each of the aforementioned methods.
For example, in method (2)(a) where perforated containers are used, it is very difficult to form a localized strengthening layer following the exact surface profile of a casting. It is also very difficult to form thin strengthening layers. This method is suited mainly for forming thick strengthening layers in thick castings.
In method (2)(b), a magnetic field that generates a pre-determined configuration following the profile of the casting surface is required in order to hold the special material. Thus, different configurations of the (electro-) magnets are required for different casting designs, which is impractical and costly to apply for the production of frequently changing casting designs. When the local profiles of the casting surface are complex, generating appropriate magnetic fields to hold the special material in the desired area and in uniform thickness becomes difficult. In addition, only ferromagnetic special materials can be used.
In method (2)(c), unless there is considerable difference in densities between the special material and the liquid casting material, loose particles can be engulfed in the flowing liquid casting material and swept away from the desired area. Therefore, the formation of a uniform strengthening layer on the casting surface becomes very difficult, if not impossible. It is also apparent that this method can only be applied to very simple, generally flat, casting surfaces on the bottom of the casting mould.
Method 2(d), which uses spray coating techniques, is perhaps the most versatile with respect to casting size, casting shape, choice of special material, uniformity of the coating layer, cleanliness of the coating layer. Spray techniques further permit better replication of the exact shape of the casting mould. Spray techniques are particularly useful when combined with precision casting processes to produce net shape castings, for example in fabricating high performance moulds and dies and in producing pump components. Localized strengthening on internal casting surfaces can be achieved by spray coating a ceramic/sand core followed by placing the core in the casting mould before casting. However, the major challenge for spray coating methods is to overcome the tendency for the coated layer to spall from the mould surface before casting. In addition, spray coating has traditionally provided coatings of only a very limited thickness, likely as a result of the spallation problem.
There remains a need in the art for a versatile method of modifying the surface or near surface of a casting, in particular a spray coating method which reduces the tendency of the coating layer to spall from a casting mould and which permits formation of thicker surface layers on the casting.